When Amplex Red connects with a gold catalyst the structure is changed to make a fluorescent molecule that immediately emits a flash of light, showing where the catalytic event took place. Right, electron microphoto of a single gold nanorod, encased in a poirus silica shell. The shell keeps rods from clumping together and allows experimenters to use heat to clean away a coating that forms when the rods are created.

Abstract:
Engineers trying to improve fuel-cell catalysts may be looking in thewrong place, according to new research at Cornell.

To make better fuel cells, study the defects

Ithaca, NY | Posted on February 20th, 2012

There is growing interest in forming the catalysts that break down fuelto generate electricity into nanoparticles. Nanoparticles provide alarger surface area to speed reactions, and in some cases, materialsthat are not catalytic in bulk become so at the nanoscale.

These nanoparticles, typically just a few tens of nanometers (nm) wide,are not neat little spheres, but rather jagged chunks, like microscalegravel, and researchers have found that they can correlate catalyticactivity with information about the number and type of their surfacefacets. But they may be looking at the forest and ignoring the trees.

"People measure the activity of a sample and then try to understand byusing facet information," said Peng Chen, associate professor ofchemistry and chemical biology. "The message we want to deliver is thatsurface defects [on the facets] dominate the catalysis."

Chen's research is reported Feb. 19 in the online edition of thejournal Nature Nanotechnology.

Instead of particles, Chen's research group studied catalytic events ongold "nanorods" up to 700 nm long, effectively letting them see howactivity varies over a single facet. Gold acts as a catalyst to converta chemical called Amplex Red into resorufin, which is fluorescent.

Each time a catalytic event occurs, the newly created molecule ofresorufin emits a flash of light that is detected by a digital cameralooking through a microscope. A flash typically appears as severalpixels, and additional computer processing averages their brightness topinpoint the actual event to within a few nanometers. The researcherscall the technique "super-resolution microscopy." After flooding afield of nanorods with a solution of Amplex Red, they made a "movie"with one frame every 25 milliseconds.

The researchers found more catalytic events near the middle of a rod,tapering off toward the ends and a jump back up at the ends. They alsofound variation in the amount of activity from one rod to another, eventhough all the rods have the same types of facets.

To explain the results, they proposed that activity is higher in areaswhere there are more surface defects. The nanorods are made by growinggold crystals from a small "seed" crystal, growing outward from thecenter to the ends, Chen explained, and more defects form at thebeginning of the process.

"Knowledge of the surface facets ... is insufficient to predictreactivity," the researchers said in their paper. "Surface defects …can also play a dominant role."

The findings with a gold catalyst and fluorescent molecules should beequally applicable to other catalysts, including those used in fuelcells and for pollution remediation, Chen said.

The research was supported in part by the Army Research Office, theNational Science Foundation (NSF), the Department of Energy and theAlfred P. Sloan Foundation. Part of the work was carried out at theCornell Center for Materials Research and the Cornell Nanoscale Scienceand Technology Facility, both supported by NSF.